"Good! Then we must get away at once," observed Sharpe, and within another five minutes they were being hurled along towards Hounslow, the aerodrome from which this new adventure was to begin. Forty-five minutes later a couple of S.E.9s, the fastest machines in the service, rose from the flying ground and steered a course east-south-east for the Straits of Dover. Thirty-five minutes later, the necessary signals having been accepted by the Dover patrols, with throttles wide open, the two daring young aviators rushed the Channel at one hundred and fifty miles an hour. The French patrols having been informed by Dover, permitted them to pass unchallenged. And now changing course till they steered almost due south-east, they sped onwards, catching now and again a glimpse of the old battle-front of the days of 1914-1918, where the shell-marked craters of the Hindenberg line were still visible from the air. Then they followed the railway line from Laon to Rheims, left the ancient town of Nancy to their left, and, crossing the Vosges Mountains and forests a little to the north of Belfort, they dropped down quietly to the landing ground outside Mulhausen in Alsace, as the clock in the Market Square struck the hour of noon. Having left their machines and flying gear in charge of the commandant, they entered the town, purchased a portable camp outfit, and, dressed as tourists of the pedestrian and naturalist type, continued their journey, crossed the Rhine and entered the Schwarzwald, ostensibly to study the fauna and flora of the Black Forest. "Phew! I’m tired of this load. Let us camp here for the night, by this little clearing, where these seldom trodden footpaths diverge," said Keane, some hours later, as, weary and dusty with his three hours’ tramp through the bracken and the tousled undergrowth, he threw down his heavy knapsack and nets, and began to wipe the perspiration from his forehead. Then they lit a small fire of dried twigs, cooked their evening meal, and lit their pipes. After a quiet smoke, during which time they carefully re-examined a survey map of the Schwarzwald, they began to talk in low whispers, whilst the sun descended amongst the pines on the western heights, over which they had dragged their weary feet. "It is my opinion," whispered Keane, "that we are within five miles of that secret aerodrome." His companion nodded, almost drowsily, although every faculty was kept constantly alert. "It is just possible that one of these paths leads to the very spot, but it will be necessary to explore them both. We must be extremely careful, however, for this professor is sure to prove a wily opponent. I hope, however, some wood-cutter or peasant may pass this way soon, and that we may learn something from him which will help us," continued the senior airman. "What if the wood-cutter should prove to be the professor himself?" asked Sharpe, with a humorous twinkle in his eyes. "It is even possible," returned his companion.